I'm suggesting the problem you're encountering with 'high level magic' stems from a combination of an artefact of your house-rules and the DM not being able to properly deal with 20th level PCs.
Even if your right, what’s the point of this statement, as it does nothing for the OP.
who cares why they need the help, they just do...so either provide some or get out of the thread.
maybe the dm is bad at handling 20th level, and instead of changing their whole play style and take a masters course in dming...would instead like to implement one rule that will help them.
maybe the dms old house rule is magnifying the problem...but the party likes the Houserule and so wants to keep it and add something else to compensate.
so your task is simple: your not here to give the dm lessons, your not here to explain to them why their game sucks, your here to help them understand the consequences of the Houserule presented...within the game world that has been presented.
if you don’t want to do that, then there are many other threads to reply to.
and my goodness, the fact that half the posts on this thread are about AC calculations is just ridiculous. It’s 20th level people, it could be +2 god bonus for all we know. The high level campaigns I have run all get “weird” at that point, the group is going to have so many crazy npc contacts, weird favors, special circumstances. 20th level is almost never what it is on paper. So stop trying to prove the group is playing wrong and just evaluate the Houserule